Temperature compensated tuned circuit



oRl/ 'r- //v CYCLES March 20, 1945. BELL r 2,371,790

I TEMPERATURE COMPENSATED TUNED CIRCUIT Filed Aug. 17, 1942 /z' T i 7 v w I 49 co zeafi 1 ND //5 H5475}? SUPPLY TEMPERA TURC 5,

DIV/DER ,Arla MC.

| l I I I l I l 5 4 5 s 7 a TIME-MINUTES AFTER TURN/N6 HEATER SUPPLY o/v Patented 20, 1945 l TEMPERATURE CO v CIR MPENSATED TUNED CUIT i John F. Bell, Chicago, 111., assignor to Zenith t Radio Corporation, Chicago, lll., a corporation p f Illinois Application August 1'7, 1942, Serial No; 455,140. H 24 Claims. (01. 2501-36) This invention relates to temperature compensated tuned circuits and has for its principal object to provide an improved temperature compensated tuned circuit.

The invention is particularly adapted foruse inconnection with oscillators and will be described in connection with an oscillator. It will, however, be understood that the invention is applicable to any tuned circuit associated with the operating circuit of a thermionic tube.

iWhen an oscillator tube is heated up, the frequency of the associated tuned circuitdrifts, becominglower, if the tuned circuit includes inductance and capacity of usual form. This drift may be divided into two partaone part consisting of frequency changes resulting fromchanges ofparameters and characteristics of the elements of the tube as a result of its heating up. This part of the drift occurs during an appreciable time period in the case of tubes having indirectly heated cathodes. v v

When a tube heats up, there is capacity change between any two of its electrodes because of change in at least three factors. As the tube heats up, its structural portions change sizeand the spacing between the electrodes changes, resulting in a change of inter-electrode capacity. When the dielectric structure within the tube changes, the dielectric constant of the dielectric material changes to some extent and is associated with a change incapacity between electrode connections. As the tube'heats up, its cath= ode emission increases, resulting in anincrease in electron density between electrodes and a corresponding change in interelectrode capacity by reason of this changed electron density between electrodes. l The other part of the drift consists of frequency changes resulting from heating up the other parts of the tuned circuit, for example, the capacitor and the inductor. The latter part of the drift depends upon the ambient temperature.

The first part which normally causes most of the drift, occupies about five or ten minutes,,depending upon the type of tube used. Th other 'part of the drift may be detected. for a much longer period, for example v45 or 60 minutes. The

firstpart corresponds to the heating up of the tube and the other part correspondsto theheatin up of the set as'a whole and particularlycon responds to the time necessary for the capacitor and the inductor of the tuned circuit to attain static temperature conditions. For convenience I 1 5 of and throughout its operation;

drift and the other part ofthe drift as the long term drift.

'In the broadcast range; the driftissmall enough to be negligible. However, with high frequency 35" oscillators, for-example, oscillators having a frequency of the order of megacycles, the drift referred tobecomes a very serious objection. g V

One of the objects of the inventionis to pro! vide a simple means forcorrectingboth initial and 19 long term drift of a tuned circuit.

" stunner object of the invention is to provide an improved I tuned circuitv associated: with a thermionic tube which will hold its operatingfre quencywithin very closelimits fromthe initiation A further 'object of theinvention is an improved tuned circuit in which a combination of a single heater and acompensating condenser is arranged to correct both initialand long =20 term drift.

' A further object is to provide an improved tuned circuit compensated for both initial and long term drift and having substantially cohstantfrequency over a wide range of heater supply voltage.

Other objects, advantages and capabilities of the invention will appear from the following description of a preferred embodiment taken in conjunction with the accompanying drawing, in

which:

,Fig. .1 is a wiring diagram of anoscillator embodying my invention; and I p a Fig. 2 is a set of curves showing the drift of an oscillator with respect totime for. Various adj Stments of the compensating elements. Y

, Referring to the drawing, the reference numeral l0 designates a thermionic tube. The tuned circuit inductance. is designated ll andthe tuning.

capacitance is provided by two capacitors l2 and 4o 13. A single capacitor maybe employed instead of the two capacitors Hand .13 but for reasons which will hereinafter appear I prefer to employ two capacitors in this relation.

. The two capacitors l2 and l3 havetogether an i effective capacity coeificientwhich compensates the circuit for changes in the ambient temperature. In order to provide this correction, it is necessary to determine'the residualiCo), capacity of the tuned circuit,that is, the capacity other than thatprovided by the capacitors l2 and J3.

. To. determine the. residual. capacityQI. remove the capacitor 13 and. replacethe capacitor l2 by a capacitor oflknown value (C1)} and determine the oscillatorfrequency :(F1).. I repeat the refer to the first part of the drift as the initial testlwith -a different known capacitor (C and to provide a thecapacity I again determine the frequency (F2). The residual capacity is known from the equation:

F22C2-'F12C1 "*FTFF Then I substitute for the capacitors I2 and I3 a capacitor having a capacity C: mfarads) equal to the sum of the capacities of [2 "and 13, this.

capacitor having a known temperature 'coefilcient of K3 ,zmfaladS per farad per degree centigrade. The temperature coefiicient is considered negative when increase of temperature causesa decrease of capacity. I make two frequency determinations at two different ambient temperatures, allowing sufiicienttime' 'for the s'et'to at tain constant temperaturesin'both case's. "From the twofrequency determinations I find the circuit thus constituted has a drift or df cycles per "tained with the lead of the resistor substantially larger than the lead of the ceramic capacitor.

degree centigrade. df is considered to be positive if an increase of temperature produces'an increaseeof frequency; and negative if s n-increase of temperature produces a decrease of frequency. I This drift would be compensated by increasing a b. perdegreecentigfade temperature rise. Both capacities areiexpres'sed in iiarads. If, as in most cases, d1 is negative, 'the'ca'pacity' C o-I 02 is actually, reduced. ,Sincethe capacitor C3 increases theitc tal capacity by CaKs per degree centierade I temperature .rise, fthe'total increase of capacity (miiar'ads) per degree centigrade necessary "for correcticnicr ambientitemperature'is I l Lemma-1..

Alitermsofthis expression are known. I

Imay employ a single capacitor C3 having a temperature coeiiicient Kx such that CsKsz is'eq'ual to the last expression and have the circuit compensated for ambient temperature.

o'f"the'j'capacities C12 and Cubeing equal to "C3 and having temperature coefficients K12 and K1:

- respectivelyso that,

- Thus if I find thatbiKx should be equal {to -,.0133 farad per degree 'centigrade temperature rise where-Cris "100 praradsfrma use as capacitor I2 a ceramic capacitor-having a capacit'y'o'fbll uufarads anda coe'fiicient-of- -p00l55 ifarad per mifarau per degree centigrade; and I "may use as capacitor 13 a cer'amic capacitor having 9. caparity of 40 fa'rads and having a oeflicientof 110011 and thus have correct capacity andcorrectcompensationfor-ambient temperature The temperature coeflicient 'of the capacitor I3 may be selected at any value suitable Yior'correctio'n o'finitial drift and the'temperature coefii'cient of the capacitor I2 may-be selected to give the necessary change 'ofcapacity for coi're'ction for ambient temperature. Usually 'CsKs isiieg'ative, butin somecase's it is positive. In the latter case,

I prefer, however, to 'usetwo ca acitors I'Zand lathe-sum volta e.

V The conductor I5, is arranged so that it serves -as'a temperature divider, that is, it is a heat con- "ductor analogoiis'to the slide wire of a voltage divider, temperature being regarded as analogous to In otherwords, I provide for a temperature gradient along the conductor I5. At its upper'end (as shown in the drawing) the temperature of the conductor 'l5 isat amaximum. Atzits'lower'end the temperature is substantially that of the chassis'lil. The chassis'provide's a considerable mass of material so-that itmaintains the lower end of the conductor lisubstantially :atchassis temperature, which is substantially ambient temperature. Intermediate-points of the conductor I5 have intermediate-temperatures which increase from the lower to the'upper end, as viewed in the=drawing= The position of the pointer connection I9 on :the conductorIS determines the maximum temperature which the capacitor I3 attains, and the length of the .relativelysmall conductor I4 determines primarily the rate at which the capacitor I3 heats up when the switch [I is closed.

' When the switch I1 is closed, the cathodeoi the tube! warms up almost-immediately and in a matter of five or ten minutes the elements "I of =the'tube I 0 have reached static temperature K12 should be positiveand'the relation of the last equatibnshouldbe preserved. The initial drift is *crdinarily negative and Kn is correspondingly negative;

One side of each 'of'the capacitors I2 and "I 3 is cfonnected'to theplateoithetubelll. The other side of'thecapacitorf I 2 is' roundedtothe chassis. The other side of the "capacitor '13 has its lead HI connected at a 'pointf1 9ftotheflead I5 r are sister It: The lead I5 issoldere'd to 'the chassis conditions' The resistor I6 likewise heats up almost as quickly as the cathode. There is a slightlag due to theheat capacity of the material 0f .the conductor I5,but this lag is so small compared with thelength of time required for the elements of the tube [0 to acquire their-static temperatures, that it may be neglected altogether. Thus immediately the switch I1 is closed, the point-I9 of the conductor I5 heats up rather quickly but its static temperature is determined primarilyby its locationbetween the chassis 18 and the heater It. The lead l4, being relatively small, has a definitely-limited heat transfer capacityyand'since the compensating capacitor I3 hasan appreciable mass and moreover is subjectto loss of heat through its other 'conductonthere is a certain time lag in' the heating up 0f the capacitor I3 'to its static condition. This lag corresponds to the lag of the temperature-rise in the elements of the tube Ill. The lag may be adjusted by changing the material of the conductor or its cross-sectional area. Ixprefer, however, toadjustthis lag by adjusting the efiectivelength'oi the conductor I4. Just as a long piece of resistor wire has a high resistance, so :a

relativelyl'ong conductor I'4 has .aLhigh thermal resistance and; will transfer heat more slowly than would a short conductor. Thus,*the length of the lead "I l is correlated to the tim'elag ofthe heating ofthe elements of the tube "I 0 after the switch i! is closed. "It-is to beinoted that the conductors I l and 15 have completely negligible electricalresistances, h

The final temperature which the capacitor '13 will attain, depends primarily-upon the position of the point IS on the conductor I5. Ifthepoint ll] is high (as viewed in the drawing), the ultimate temperature of the capacitor I3 is greater, and vice-versa. Movement of the point [9 up wardly or downwardly has aslight effect upon the rate of heating of the capacitor l3, but its effect is very slight and can normally be corrected by a very slight change of the length of the conductor l4. 4 if I The inventionwill more readily be understood bythe following specific example of an oscillator embodying my invention. The heater voltage is 6 volts. Theresistor l6 has aresistance of 19 ohms. The conductor I is a No. 16 wire approximatelyone inch long. The length of the conductor [4 which is a No.22 wire, is approximately one-half inch,*and it is connected to the conductor at a point approximately from the heater [6. v l

D enables an oscillator to be turned on and utilizedimmediately the cathode warms up to give immediate reception or transmission with drift so small as to be completely negligible.

While I preferto determine the correct positionof the point l9 and the length of the conductor tend to cancel each other in most cases and those cases which are not compensated to suflicient degree of accuracy; can readily be found by test and While the correct position of the point 19 and the'correct length of the conductor l4 may be determined precisely by calculation on the basis of the thermal capacity of the capacitor l3, its radiation and conductive losses, its heat capacity, etc., it is far more convenient in practice to determine the length of the conductor l4 and the position of the point l9 experimentally in the first instance. Tooperate in this manner, 1 connect the lead l4 to a point IS on the lead l5 and determine the initial drift when the switch I! is closed. Since the circuit haslbeen' compensated for, ambient temperature variations, the drift; if any, is due exclusively to changes in the tube ID as a result of its increase of temperaturedue to the heater.

In Fig. 2 curve A shows the drift in cycles of an oscillator tuned to 16 megacycles and compensated for ambient temperature changes but not compensated for initialdrift. It is to be noted that in the first six minutes, the frequency driftslower until a total drift of" about 3500 cycles is attained, 'whereafter the frequency remains substantially constant,

With the circuit of Fig. 1, I may obtain a curve of the general type shown in curve B. It ishere to be noted that in the first couple of minutes the frequency increases rapidly and then drops rapidly until at the end of about six minutes, the frequency becomes relatively constant. The driftof two minutes which may amount to about 2000 cycles, indicates that the capacitor I3 heats up too rapidly, whereas the final drift of 1000 cycles indicates that the final temperature of the capacitor [3 is too high. To correct the curve B, I do twothings: First I move the point l9downwardly away from the heater l6 and I lengthen the conductor M. The first adjustment brings the final part of the curve down- Wardly in the manner shownin curveC and the other adjustment reduces the initial hump on the with the curve D of Fig92and intended primarily ductor l'5, the same oscillator had a total drift of corrected by slight change ofposition of the point l9 and/or a slight variation in the length of the conductor l4. 1

One remarkable attribute of my improved tuned l circuit is that it has a very slight variation of frequency with change of heater supply voltage. Thus, with an oscillator tuned and compensated for a frequency of 16 megacycles in accordance cycles, the frequency change-at 6 volts was +40- cycles, and the frequency change at '7 volts was +80 cycles. 1 l

With the heater [6 disconnected from the con- 1900- cycles when the heater supply was varied between 5 and '7 volts. l l

Consequently, my improved compensated tuned circuit enables me not onlyto provide for instant communication without appreciable drift immediatelyafter the tube cathodeheats up, but also enables me to use as the heater supply the battery curve. The low final frequency of curve C indicates that the final temperature is too low and I move the point 19 upwardly. At the same time I reduce the initial hump' on curve C by lengthening conductor l 4 to a slight extent. I thus obtain curve D which correspondsto excellent compensation since the drift .is less than 500 cycles in 16,000,000. I may, however,'greatly improve the curve D by continued small manipulations of the position of the point 19 and the length of the conductor l4. Curve D is, however, regardedas indicating perfect compensation for all practical purposes. Compensation of the nature of curve charging system of an automotive vehicle in which the voltage varies considerably, without creating appreciable drift due to variation of the ch'arging or battery voltage.

, Although the invention has been described in connection with the details of a specific embodiment thereof, it must be understood that such details are not intended to'be limitative of the invention, except in so far as set forth in the accompanying claims. l

Having thus described my invention, I declare that whatIclaimis:

1. A temperature compensated tuned circuit 5- ,having capacity and inductance connected in an operating circuit of a thermionic tube, a member arranged to heat up simultaneously with the tube, a thermal conductor havingone end connected to said member and the other exposed to ambient temperature, said capacity including a capacitor having a negative temperature coefiicient, and a thermal conductor thermally connected to said capacitor and to an intermediate point on first said conductor.

2. A temperature compensated tuned circuit having capacity and inductance connected in an operating circuit of a thermionic tube, a heater, means for supplying energy simultaneously to said heater and to the heater'of the tube, a thermal conductor connected tosaid heater and to the chassis, said capacity including a capacitor having a negative temperature coefficient, and a second thermal conductor connected between said cahaving capacity andninductance .connectedin an operating circuitzof axthermiom'c tube a member arranged-to heat up simultaneously with the tube,

a thermal conductor having one, end ;conneoted to said member and the other exposed to ambient temperature, said capacity including a capacitor having a negative temperature coefiicientpand a thermal conductor (having a relativelyhigh 1ther-, mal resistance connected between said capacitor and a pointalong firstsaidthermal'conductor.

4. A temperaturecompensated .tuned circuit having capacity andinductance connected in an operative circuit of .a, thermionictube, a heater,

means'for supplying energy simultaneously to said asrmao 3.. A temperature compensated tuned .fcircuit compensate for-drift of frequency resulting-from change of ambient temperature, and athermal conductor connected to said capacitor-and to 1 an intermediate point of firstrsaid thermal conductor,

bination,a resistorlheater connected tothe heater supply of the tube sothat said heaters warm-up together, a thermal conductor extending from said resistor heater to tbe warmed up thereby, said conductor being arranged'to'establish a'temperature gradient therealong, wsaid capacitor means includinga capacitor having anegativetempcrature coefiicient, and a second -thermal conductor connected to'said capacitor and toanintermediate point on first said thermal conductor.

'7. In an oscillator comprising a thermionic tube and a tuned circuit including capacitor means and inductormeans connected torthe tube, in combination, a resistor heater connected [to thehea'ter supply of the tube so that said heaters warm up together,-a thermal conductorextending from said resistorheater tobe warmed up thereby, said conductor being arranged -to establish a temperature gradient atherealong, said capacitor means including'a capacitorhaving a negative temperature coeflicient of sufficientmag nitude to compensate for frequencydriftof-the circuit with change of ambient temperature, and a second thermal conductor connected -to;;said capacitor and to an-intermediate point "on'first said thermal conductor, the pcint'of connection of said thermal conductors being located toeifect heating up of the capacitor tora degree-sufficient to compensate for frequency drift due to the heating up of the tube .elcments andthe eifective length of the second thermal :conductorlbes ing-arranged toprovide heat resistance suiiiclent to limit the rate of heating up "of :.the capacitor to align substantially the: change :of capacity of the capacitor with the change. of the characteristics of the tube-during thetubeheating period.

.-:8. i-A temperature compensated Ltuned circuit comprising a pair. ofwcapacimrsand inductor means connected in an operating circuit of ,a thermionic tube, .said capacitors having temperature .coeflicientsrrof magnitude andsign to compensateifor frequency drift. due to change of ambient temperature, a heater, means for supplying energy simultaneously topsaid. heater and to the the tube, the point of connection between said thermal conductors being arranged to supply heat'to last said capacitor at a rat and-to an extent to compensate for drift of frequency due to heatingup of thetube. I 3 '9. A temperature compensated tuned circuit comprising a capacitor andinductor connected in-an operating circuit ,of a thermionic tube, a

heater, a source offvariable voltage, switch means forsimultaneouslyconnecting said source to said heaterand to-theheaterof the tube, a thermal conductor connected to firstjsaidheater and arranged to establish a temperature gradient therealong, said capacitor :having. a temperature c0- efiicient adapted to 'correctfrequency drift due to heating up of the tube, and a second thermal conductor connected, to said capacitor and to anintermediate point on'first said thermal conductor, the point of connection being arranged to supply heat to saidcapacitor at a rate and to a rfinal capacitor v.te r l trature whereby the change of capacityof the capacitor compensates for the frequency drift du to heating up of the tube. a i

10. A temperature compensated tuned circuit comprising a capacitor and inductor connected in an operating circuit ofathermionic tube having 'an'indirectly heatedcatho'de, aheater resistor, means 'for supplying current simultaneously to the'heater of the tubean'dto 'saidheater'resister, a thermal conductor connected to first said heater and arranged toestablish a temperature gradient thereailong, saidfcapacitor having a'temperature coefi'icient adapted to correct fre- 55 quency drift due to "heating up or the tube, and asecond thermal conductor 'conn'ected'to said capacitor andto an intermediate point on first said thermal conductor. j

ii. A temperature "compensated tuned circuit comprising a capacitor and inductor connected in an 'operatingcircuit ofathermionic tube having a predetermined "rate of temperature rise upon 'energizaticn, said capacitor and inductor having a smaller predetermined rate of temperature rise than-said tube upon 'energization of said tube, said capacitor having a temperature coefficient of capacity 'changesuch that its temperature variation tends to compensate for frequency .change due t'o' a temperature variation in :the same direction bf *said *tube and inductance, means forgenerating heat, upon energizetion of said'tube,aheatconducting path for the continuous fiow of heat from said heat generating means, :rmeans :ior stransferringheat from a point intermediate "the tends of isaid heat contemperature.

ducting path to said capacitor, said point being so chosen that said capacitorattains afinal equilibrium temperature of suchmagnitude as to correlate the operating condition 'ofsaid tube,

capacitor, and inductor after they reach a constant temperature, said heat transferring means having such a characteristic that the rate oflheat flow to said capacitor is at a rate such that the correlated operating conditionfiof said tube, ca-

pacitor, and inductor is constant during their heating aswell as after reaching equilibrium,

1 2; A temperature compensated tuned circuit having capacity and inductance and including a reactive element having a negative temperature qcoemcient, connected in an operating circuit of heater of the tubeand to said resistor, said tuned circuit including' a reactive element-having a temperature coeificient of magnitude and signto conipensatefordrift of frequency resulting from change of ambient temperature, and a thermal conductor connected to said reactive ele- -ment and to an intermediate pointof' first said thermal conductor, the point of connection being locatedto control the supply and rate of supply of heat from said heaterresistor to said reactive element to compensate for frequency drift resulting from heating up of the tube.

14. A temperature compensating arrangement for an electron discharge device having a heated electron emittingelectrode anda cold electrode, comprising a reactance connected between said temperature and change in temperature of said heated electrode, a source of heat arranged to heat up simultaneously with said-heated electrode, a heat conducting path for the continuous flow of heat from said heat source, means for transferring heat from a point intermediate the ends of said heat conducting path to said reactance, said point being so chosen that said reactance attains a final equilibrium temperature of such magnitude that the change in capacity electrodes, thecapacity between said electrodes being subject to change upon change in ambient conducting path to said' reactance, said point being so chosen that said reactance attainss a final equilibrium temperature of such magnitude that the change in capacity betweensaid electrodes with temperature is substantially compensated, said heat transferring means having such a characteristic thatheatflows to s'aidmeactance from said resistor at a rate such that heating of said reactance causes it substantially to competisate for capacity change between said electrodes during initialheating ofsaid heated electrode;

16. A temperature compensating arrangement for an, electron discharge device comprising a reactance connected in an operating circuitfbf the device, a heated-arranged to heat up upon energization of thejdeviceg-afirst thrmal cohductor having one end connected" to said heater and the other exposed to ambient temperature,

and a second thermalconductorthermally con- 'nected between said reactanceand-an interme diate pointon said firstethermal conductor, said reactance having a temperature coefficient of reactance change adaptedto' compensate for the long term reactance change *of said'electronj discharge device due to the change of 'ambient temperature and'als'o forthe initial reactance change due to the'heatiiig upof the device.

17. A temperature compensating arrangement for an electron dischargefdevice comprisingfa reactance connected in a'n operating V circuit of the device, a heater arranged toheat jup' upon energization of the jdeviceg a first thermal conductor having one end connected to said heater and the other exposed-toambienttemperatuife,

and a second thermal conductor 1 connected-"between said reactance and aniintermediate point on said first thermal "conductor, said reactance having a temperature jcoehicient: 'of' reactance change adapted tocornpensate forfthe long term reactance change oi saidelect'ron,dischargedevice due to change of ambientftemperatureandthe point of connection of said secondthermal con-fductor to said first thermal conductor being lo' catedto control the degree'of temperature change of said reactance resultingffromthejheating'ul ,o i said electron discharge device to compensate heating;

for, initial reactancefchange' es tingfromsaid 18.A temperaturecompensating arrangement for an electron discharge device comprising a reactance connected in the operating circuit of the device, a heater arranged to heat up upon between said electrodes with temperature is sub- I stantially compensated, said heat transferring means having such a characteristic that heat flows to said reactance from said heat source at a rate such that heating of said reactance causes it substantially to compensate for capacity change between said electrodes during initial heating of said heated electrode.

15. A temperature compensating arrangement for an electron discharge device having a heated electron emitting electrodeanda cold electrode, comprising a reactance connected betweensaid energization of the .device, a first thermal conductor having one end connected to said heater and the other exposed to ambient temperature, and a second thermal conductor thermally connected between said reactance and an intermediate point on said first thermal conductor, said reactance having a temperature coefficient of reactance change adapted to compensate for the u long term reactance change of said electron diselectrodes, the capacity between said electrodes being subject to change upon change in ambient temperature and change in temperature of said heated electrode, a resistor arranged to heat up simultaneously with said heated electrode, a heat conducting path for the continuous flow of heat from said resistor, means for transferring heat from a point intermediate the ends of said heat charge device due to the change of ambient temperature, the efi'ective length of said second thermal conductor being arranged to provide heat resistance sufiicient to limit the rate of heating up of said reactance to aline substantially the reactance change of the reactance with the reactance change of theelectron discharge device due to change of the parameters and characteristics of said device during its heating period. 7

19. A temperature compensating arrangement for an electron discharge device comprising a reactance connected in an operating circuit of the device, a heater arranged to heat up upon energization of said device, a chassis upon which said: discharge --device is; mounted; a first thermal I conductor" having one: end connected to said heater andsthe. otherrendaccnnected ;to said chassis;-;whereby. saidiothe'r end 151' exposed; .to: ambient temperature, and. a: second: thermal conductor thermally connected between said Area'ctanceand M .anlintermediate point on said; first: thermal conductor; said: reactance having a: temperature 00- eflicient of reactancei change: adaptedto: compensate: fOlTthei long: term, reactance change of said electron: discharge; device. due: to the; change. of ambient: temperature: andalsm for. the initial I reactancezvchan e due-to: the" heating up of the device.

20.1111 9;; radio system arranged. to maintainza.

characteristicfofjsuch system responsive to; temperaturelchangest in apredetermined manner, the combination. or a" source ofheat producedrini responses to :operation of l the? system, a: temperature divider-having one: end: in thermal contact with said heattsourceior-the:flowoi heat therefrom,

an impedance elementconnected v electrically in said; system: andhaving impedance changeable with temperature in such a manner as to affect a characteristicrof said systemin; a predetermined manner, :and a heat aconductor. of predetermined characteristicsdisposed between: a point intermediate. the endsoi said. divider and said impedanceand arrangedrtotransfer heatat such :a rate from said point, to said impedance element that the element istmaintained-at a substantially constant-operating temperature in. steady opera- -tion of thesystemiataconstantambient temperamg 510w electrica-L resistance: whereby; said. ca-

pacitor: iscelectrically connected to the chassis therethrough-.- 1 l i '22. A; temperature compensated tuned. circuit having: capacity and: inductance. connected in. an :operatmgtcircuit-zofiathermionic tube, a member arrangedxtoheatup simultaneously, with the tube, a a second member subject toi temperaturechange in response: to changein ambient temperature, a thermal. conductor-"connectedbetween saidmemhere: for; the-flow of heattherebetween, said ca pacity including a capacitor havingacnegativa temperature coeflicient; and a-thermal conductor h rm l lfi connected; to said. capacitor and .to. an

intermediate-point onlsaid;first .conducton; U

23; A: temperature compensated tuned-circuit having capacity andinductance; connected in an operating circuit of althe'rmionic tube, a member arranged to heat .up simultaneously with the tube, a second member; subject to, temperature change a in: response tmchange in ambientitemperature, a

thermal conductor connected, between said members for the flow of; heattherebetween, saidcapacity includinea: capacitor having. a negative ture. andat such varying. temperatures inv tran- .sient-operationlof, the system; as to. maintain. a substantiallylconstant operating-condition of-said sy'stemat a constant ambientitemperature. r

21'." A temperature, compensated. tuned'l circuit having capacity and'lin'ductance connectedlin an operatingv circuit. of! a thermionic tube having a heater a chassis,.a heat; source, meansior supplying energy simultaneously, to said heat source andlto. the. heater ofv the tube, a thermal. con,-

ductor connectedlto. saidlieatsource and. to the chassiatsaid capacity including a capacitor having, a negative temperaturelcoeflicient, and a.sec.- end-thermal: conductor connected betweensaid capacitor and an intermediate point on saidlfi'rst thermal conductor, said thermal conductors havtemperature coefiicient, and. athermal. conductor thermally connected tosaid capacitor and to an intermediate point on said first conductor, said thermal conductors having low'electrical resistance; whereby said i capacitor is electrically connected to the a second member. therethrough,

- 24: In apparatus for controllingwelectri'cal operation of: a radio system subject to varying opcrating. conditions in' response to. varying temperatures; v said system including a compensating temperature responsive. impedance element, the

combinationcomprising means for. producing heat simultaneously withv operation of the system; a

metallic. connection having resistanc to the flow of heat, means for transferring. heat from said heat producing-means vthrough said metallic connectioniwhereby) atemperature gradient is producedtherealong, andimeanstincluding a second metallic connection, for transferring heat from a pointaintermediate the: end. of vsaid first metallic connection to said compensating element at a .predetermined rate suchthatla compensating eiiect is produced on said systemto reduce the effect ofltemperaturechange. l t

, 1 JOHN F.BELL..- 

